The present invention relates to an optical recording medium and in particular, to an optical recording medium of a writable type using an organic pigment as a recording material.
With the advent of information-oriented society, a request for a large-capacity memory for recording a great amount of information such as image, voice, and data is becoming stronger and stronger.
In order to answer such a request, a disc-shaped optical recording medium has a large recording capacity and a high reliability as recording and reproduction are carried out in a non-contact state. Besides, such a disc-shaped optical disc has various other merits that it can easily be carried and produced at reasonable costs in a mass production, and has been widely spread up to now.
As a recording material of an optical recording medium, the have been proposed various materials: rare earth-transition metal amorphous alloy thin film such as Tbxe2x80x94Fexe2x80x94Co, a phase change material such as Gexe2x80x94Sexe2x80x94Te, an organic pigment material such as cyanine pigment. Among these materials, the organic pigment is used in a writable optical recording medium on which a user can write in once. This material has no problem of corrosion and is low in toxicity, which is a merit that only a small environmental load is caused.
As a specific configuration example of an optical recording medium using such an organic pigment, there can be exemplified the following.
Firstly, there can be exemplified an air sandwich configuration consisting of two light transmitting substrates each having a recording layer containing an organic pigment and bonded so as to oppose the recording layers to each other and to form an air layer between the substrates. This air sandwich type optical recording medium is available in market for data recording.
After this, the Proceeding of SPIE, vol. 1078, page 1078 discloses a configuration having a recording layer containing an organic pigment, an optical reflection layer, and a protection layer successively formed on an light transmitting substrate, i.e., a normal compact disc configuration having a recording layer containing an organic pigment. As this optical recording medium has a high reflection ratio of 70% or above in a wavelength band of 780 nm used in a compact disc, after recording it is possible to obtain a signal characteristic compatible with a compact disc on market. This optical recording medium is used for music recording, video recording, and a personal computer data recording and now is forming a large market.
Moreover, in the field of optical recording, besides such a recording material and medium configuration, study has been made on the optical system for recording.
That is, in the optical recording medium, a laser beam is used for recording and reproduction.
In recording, a laser beam is focused on a recording layer and within the laser spot, an optical change is generated in the recording layer so as to form a pit. Moreover, in reproduction, a laser beam is focused on the pit so as to detect a difference between the reflection ratios of the pit and a portion not having the pit. The recording density on such an optical recording medium is determined by a laser beam spot diameter. As this spot diameter is reduced, it is possible to record with a higher density.
On the other hand, the laser beam spot diameter is proportional to the L/NA of the optical recording/reproduction optical system (wherein NA is a numerical aperture of the objective lens and L is the wavelength of the laser beam). Consequently, the recording density of the optical recording medium is determined by the numerical aperture NA of the objective lens used in this optical system and the wavelength L of the laser source. As the NA increases and the L decreases, the recording density can be increased.
For this, recently, a study has been made so as to reduce the wavelength of the semiconductor laser as a laser source. For example, as reported in xe2x80x9cO plus Exe2x80x9d, vol 199, page 71 (1996), there is a trial to apply a semiconductor laser having a wavelength band of 630 nm to 650 nm for optical recording.
However, when reducing the wavelength of the laser beam for recording and reproduction, it is necessary to select a recording material matched with the wavelength band. An organic pigment is selected because of having a sufficient optical absorption and reflection ratio in the wavelength band of the laser beam for recording and reproduction.
For this purpose, for example, organic pigments to be used for 630 nm to 650 nm are disclosed in Japanese Patent Laid-Open 6-40161, Japanese Patent Laid-Open 6-40162, Japanese Patent Laid-Open 6-199045, and Japanese Patent Laid-Open 7-186530.
However, these organic pigments do not have a sufficient optical absorption or reflection ratio in the range from 630 nm to 650 nm and have a problem in the recording sensitivity and the signal modulation degree.
It is therefore object of the present invention to provide an optical recording medium showing a high reflection ratio and an appropriate optical absorption in the short wavelength range of 630 nm to 650 nm and enabling a preferable recording and reproduction characteristic.
In order to achieve the aforementioned object, the inventor studied and found that a polymethine pigment having 1 to 4 carbons in the methine main chain and an atomic groups including a predetermined aromatic ring at both ends enables a high reflection ratio and an appropriate optical absorption in a wavelength band from 630 nm to 650 nm.
The optical recording medium according to the present invention has been proposed according to the aforementioned finding and includes a recording layer and reflection layer formed on a light transmitting substrate, the recording layer formed as (1) a pigment layer containing at least one of a pigment compound a, pigment compound b, and pigment compound c; (2) an organic pigment layer containing at least two of the pigment compound a, the pigment compound b, and the pigment compound c; (3) an organic pigment layer containing at least one of the pigment compound a, the pigment compound b, and the pigment compound c in combination with at least one of a pigment compound d, a pigment compound e, and a pigment compound f; (4) an organic pigment layer containing at least one of the pigment compound a, the pigment compound b, and the pigment compound c in combination with a pigment compound g, a pigment compound h, a pigment compound i, and a pigment compound j; (5) an organic pigment layer containing at least one of the pigment compound a, the pigment compound b, and the pigment compound c in combination with at least one of the pigment compound d, the pigment compound e, and the pigment compound f in combination with at least one of the pigment compound d, the pigment compound e, and the pigment compound f; (6) an organic pigment layer containing at least two of the pigment compound d, the pigment compound e, and the pigment compound f; or (7) an organic pigment layer containing at least one of the pigment compound d, the pigment compound e, and the pigment compound f, and pigment compound j in combination with at least one of the pigment compound d, the pigment compound e, and the pigment compound f.
These organic pigment layers enable a high absorption degree and a reflection ratio in the wavelength band from 630 nm to 650 nm and show a small wavelength dependency in this range. Consequently, the optical recording medium having a recording layer made from these organic pigments enables to obtain a high recording sensitivity and a signal modulation degree in the wavelength band of 630 nm to 650 nm. 
(wherein Xxe2x80x94 represents Brxe2x80x94, Ixe2x80x94, ClO4xe2x80x94, BF4xe2x80x94, PF6xe2x80x94, and SbF6xe2x80x94. Moreover, A1 is an atomic group expressed by Chemical Formulae 136 to 138, and B1 is an atomic group expressed by Chemical Formula 139.) 
(In Chemical Formulae 136 to 139, R1 represents an alkyl group, alkoxy group, and aryl group. When a molecule contains a plurality of R1, the R1 may be either identical substituents or different substituents from one another. Moreover, R2 and R3 represent an hydrogen atom, alkyl group, alkoxy group, amino group, nitro group, cyano group, aryl group, alkoxycarbonyl group, sulfonylalkyl group, and halogen atom. When a molecule contains a plurality of R2 or a plurality of R3, the R2 or R3 may be identical substitutens or different substituents from one another n is 1 or 2; and m is 1, 2, 3, or 4.) 
(wherein Xxe2x80x94 represents Brxe2x80x94, Ixe2x80x94, ClO4xe2x80x94, BF4xe2x80x94, PF6xe2x80x94, and SbF6xe2x80x94. A2 is an atomic group expressed by Chemical Formulae 140 to 143, and B2 is an atomic group expressed by Chemical Formula 144. Moreover, Y1 is a hydrogen atom, alkyl group, or halogen atom.) 
(In Chemical Formulae 140 to 144, R4 represents an alkyl group, alkoxy group and an aryl group. When a molecule contains a plurality of R4, the R4 may not be identical substituents. Moreover, R5 and R6 are hydrogen atoms, alkyl group, alkoxy group, amino group, nitro group, cyano group, aryl group, alkoxycarbonyl group, sulfonylalkyl group, and halogen atoms. When a molecule contains a plurality of R5 or a plurality of R6, the R5 or the R6 may be either identical substituents or different substitutes from one another. The n is 1 or 2; and m is 1, 2, 3, or 4.) 
(In Chemical Formulae 145 to 150, R7 represents an alkyl group, alkoxy group and an aryl group. When a molecule contains a plurality of R7, the R7 may be identical substituents or different substituents. Moreover, R8 and R9 are hydrogen atoms, alkyl group, alkoxy group, amino group, nitro group, cyano group, aryl group, alkoxycarbonyl group, sulfonylalkyl group, and halogen atoms. When a molecule contains a plurality of R8 or a plurality of R9, the R8 or the R9 may be either identical substituents or different substitutes from one another. The n is 1 or 2; and m is 1, 2, 3, or 4.) 
(wherein Xxe2x80x94 represents Brxe2x80x94, Ixe2x80x94, ClO4xe2x80x94, BF4xe2x80x94, PF6xe2x80x94, and SbF6xe2x80x94. A4 is an atomic group expressed by Chemical Formulae 151, and B4 is an atomic group expressed by Chemical Formulae 152 and 153.) 
(In Chemical Formulae 151 to 153, R10 represents an alkyl group, alkoxy group and an aryl group. When a molecule contains a plurality of R10, the R10 may be identical substituents or different substituents. Moreover, R11 and R12 are hydrogen atoms, alkyl group, alkoxy group, amino group, nitro group, cyano group, aryl group, alkoxycarbonyl group, sulfonylalkyl group, and halogen atoms. When a molecule contains a plurality of R11 or a plurality of R12, the R11 or the R12 may be either identical substituents or different substitutes from one another. The n is 1 or 2; and m is 1, 2, 3, or 4.) 
(wherein Xxe2x80x94 represents Brxe2x80x94, Ixe2x80x94, ClO4xe2x80x94, BF4xe2x80x94, PF6xe2x80x94, and SbF6xe2x80x94. Moreover, A5 is an atomic group expressed by Chemical Formulae 154 to 159, and B5 is an atomic group expressed by Chemical Formula 160.) 
(In Chemical Formulae 154 to 160, R13 represents an alkyl group, alkoxy group and an aryl group. When a molecule contains a plurality of R13, the R13 may be identical substituents or different substituents. Moreover, R14 and R15 are hydrogen atoms, alkyl group, alkoxy group, amino group, nitro group, cyano group, aryl group, alkoxycarbonyl group, sulfonylalkyl group, and halogen atoms. When a molecule contains a plurality of R14 or a plurality of R15, the R14 or the R15 may be either identical substituents or different substitutes from one another. The n is 1 or 2; and m is 1, 2, 3, or 4.) 
(wherein Xxe2x80x94 represents Brxe2x80x94, Ixe2x80x94, ClO4xe2x80x94, BF4xe2x80x94, PF6xe2x80x94, and SbF6xe2x80x94. Moreover, A6 is an atomic group expressed by Chemical Formulae 161 to 163, and B6 is an atomic group expressed by Chemical Formulae 164 and 165. Moreover, Y3 is a hydrogen atom, alkyl group or halogen atom.) 
(In Chemical Formulae 154 to 160, R16 and R18 represent an alkyl group, alkoxy group and an aryl group. When a molecule contains a plurality of R16 or a plurality of R18, the R16 or R18 may be identical substituents or different substituents from one another. Moreover, R17 and R19 are hydrogen atoms, alkyl group, alkoxy group, amino group, nitro group, cyano group, aryl group, alkoxycarbonyl group, sulfonylalkyl group, and halogen atoms. When a molecule contains a plurality of R17 or a plurality of R19, the R17 or the R19 may be either identical substituents or different substitutes from one another. The m is 1, 2, 3, or 4.) 
(wherein Xxe2x80x94 represents Brxe2x80x94, Ixe2x80x94, ClO4xe2x80x94, BF4xe2x80x94, PF6xe2x80x94, and SbF6xe2x80x94. Moreover, A7 is an atomic group expressed by Chemical Formula 166, and B7 is an atomic group expressed by Chemical Formula 167.) 
(In Chemical Formulae 166 and 167, R20 represents one of an alkyl group, an alkoxy group and an aryl group. When a molecule contains a plurality of R20, the R20 may be identical substituents or different substituents from one another. Moreover, R21 and R22 are hydrogen atoms, alkyl group, alkoxy group, amino group, nitro group, cyano group, aryl group, alkoxycarbonyl group, sulfonylalkyl group, and halogen atoms. When a molecule contains a plurality of R21 or a plurality of R22, the R21 or the R22 may be either identical substituents or different substitutes from one another. The n is 1 or 2; and m is 1, 2, 3, or 4.) 
(wherein Xxe2x80x94 represents Brxe2x80x94, Ixe2x80x94, ClO4xe2x80x94, BF4xe2x80x94, PF6xe2x80x94, and SbF6xe2x80x94. Moreover, A8 is an atomic group expressed by Chemical Formulae 168 to 171, and B8 is an atomic group expressed by Chemical Formula 172.) 
(In Chemical Formulae 168 to 172, R23 represents an alkyl group, an alkoxy group and an aryl group. When a molecule contains a plurality of R23, the R23 may be identical substituents or different substituents from one another. Moreover, R24 and R25 are hydrogen atoms, alkyl group, alkoxy group, amino group, nitro group, cyano group, aryl group, alkoxycarbonyl group, sulfonylalkyl group, and halogen atoms. When a molecule contains a plurality of R24 or R25, the R24 and the R25 may be either identical substituents or different substitutes from one another. The n is 1 or 2; and m is 1, 2, 3, or 4.) 
(wherein Xxe2x80x94 represents Brxe2x80x94, Ixe2x80x94, ClO4xe2x80x94, BF4xe2x80x94, PF6xe2x80x94, and SbF6xe2x80x94. Moreover, A9 is an atomic group expressed by Chemical Formulae 173 to 179, and B9 is an atomic group expressed by Chemical Formula 180 and 181.) 
(In Chemical Formulae 168 to 172, R27 represents any of an alkyl group, an alkoxy group and an aryl group. When a molecule contains a plurality of R27, the R27 may be identical substituents or different substituents from one another. Moreover, R28 and R29 are hydrogen atoms, alkyl group, alkoxy group, amino group, nitro group, cyano group, aryl group, alkoxycarbonyl group, sulfonylalkyl group, and halogen atoms. When a molecule contains a plurality of R28 or R29, the R28 or the R29 may be either identical substituents or different substitutes from one another. The n is 1 or 2; and m is 1, 2, 3, or 4.) 
(wherein Xxe2x80x94 represents Brxe2x80x94, Ixe2x80x94, ClO4xe2x80x94, BF4xe2x80x94, PF6xe2x80x94, and SbF6xe2x80x94. Moreover, A10 is an atomic group expressed by Chemical Formulae 182 to 188, and B10 is an atomic group expressed by Chemical Formula 189 and 190.) 
(In Chemical Formulae 182 to 190, R30 represents any of an alkyl group, an alkoxy group and an aryl group. When a molecule contains a plurality of R30, the R30 may be identical substituents or different substituents from one another. Moreover, R31 and R32 are hydrogen atoms, alkyl group, alkoxy group, amino group, nitro group, cyano group, aryl group, alkoxycarbonyl group, sulfonylalkyl group, and halogen atoms. When a molecule contains a plurality of R31 or R32, the R31 or the R32 may be either identical substituents or different substitutes from one another. The n is 1 or 2; and m is 1, 2, 3, or 4.)